1. Field of the Invention
With the recent trend toward biological pest control, considerable attention has been given to the insecticidal properties of the bacterium Bacillus thuringienis. Of particular interest is the subspecies, kurstaki, because of its broad and effective entomocidal host range. The subspecies is commercially prominent for biological control of lepidopteran pest insects.
The sporulating cells of B. thuringiensis each produce a spore (endospore) and a diamond-shaped proteinaceous crystal (paraspore or inclusion body). The entomocidal properties have been attributed solely to the .delta.-endotoxin which is a major component of the parasporal crystal. When the crystal solubilizes in the insect gut, it gives rise to a protoxin which is activated by proteolytic digestion.
At the completion of sporulation, the autolyzing cells release both the spores and the crystals into the culture medium. For most investigative research purposes, it is desirable that the latter be recovered in a substantially pure state. Also, environmental considerations dictate that commercial insecticidal preparations be substantially free of viable spores. However, the similarity in size and density renders separation of the crystals and spores both complicated and laborious. This invention relates to a method for the production of parasporal products which simplifies, or even obviates, such a separation.
2. Description of the Prior Art
It had been known from the teachings of Shieh et al., U.S. Pat. No. 3,758,383, that it was possible to select a mutant strain of B. thuringiensis which could be cultured under sporulation conditions without the usual production of the crystal bodies. Attempts at the reverse situation have led to reports of other mutants which fail to sporulate but which do form the crystalline paraspores. NishiitsutsujiUwo et al. [J. Invert. Path. 25: 355-361 (1975)] shows the selection of five completely asporogenous strains from mutagenized wild types. Two of these were derived from the subspecies kurstaki, but overall were somewhat less toxic than the parent strain. Other attempts to mutate B. thuringiensis have led to strains which produce neither spores nor crystals [A. A. Yousten, Can. J. Microbiol. 24: 492-494 (1978)].